To meet the requirements of the EU's RoHS (Reduction of Hazardous Materials) legislation, Pb-free solder technology is being well implemented in consumer electronics and mid-range electronic systems, however, several technical issues have been recently identified in the first level interconnect structure, such as chip-to-package interactions (CPI), e.g., ILD (Interlayer Dielectric) delamination or cracking (or “white bumps”) in back end of the line structure (BEOL) of the chip during initial chip joining, and electromigration (EM) in flip-chip joints under high current density.
The white bump issue is largely due to the inherent high strength of Sn—Ag—Cu (SAC) and SnCu (SC) solders combined with the fragile dielectric materials used in the back-end-of-line (BEOL) structure of the advanced complementary metal-oxide-semiconductor (CMOS) chip design. The white bump issue has been successfully addressed by reducing Ag content in SAC solders combined with an improved design of the BEOL structure. However, in doing so, the EM resistance of flip-chip solder joints is greatly compromised in low-Ag, Sn-rich solder bumps. In our recent EM studies it has been found that the metallurgical (or microstructural) factors are crucial in determining the EM performance, for example, Sn—Ag solders provide a better EM lifetime than Sn—Cu solders. Cf. M. Lu, D-Y Shih, P. Lauro, C. Goldsmith, and D. W. Henderson, “Effect of Sn grain orientation on electromigration degradation mechanism in high Sn-based Pb-free solders,” Appl. Phys. Let. v. 92, 211909 (2008).
A fundamental issue owing to the anisotropic properties of Sn single crystals is found to be responsible for the premature failures under high current EM tests. Lu et al., supra. The solute atoms such as Cu, Ag, or Ni are known to transport much faster along the C-axis than a- or b-axis of the body centered tetragonal Sn single crystal structure. Since a typical flip-chip solder joint consists essentially of only a few Sn crystals, the anisotropic properties of Sn single crystals critically affect EM performance and other physical/mechanical properties when Sn crystals are oriented in a less favorable direction. The high Pb, flip-chip interconnection performs much better in EM tests than Sn-rich solders, since a Pb crystal is isotropic and Pb has a much higher melting point (328° C.) than Sn (232° C.).
In order to control the microstructure of Sn-rich solders and thereby to improve EM performance and other properties, minor alloying additions to Sn-rich solders (such as Ni, Zn, Ti, Sb, Bi, and others) have been extensively investigated. Among them, Zn addition was found to be the most effective in enhancing EM performance with other beneficial effects. The microstructure study of Zn-doped solders has revealed that minor Zn addition stabilizes the microstructure of Sn-rich solders during high temperature aging as well as EM tests.
A reaction barrier layer in the UBM structure is important to the reliability of C4 solder joint. A good UBM needs to be wet well by the solder and form stable, but not too many, intermetallic compounds at the interface during solder reflow. To meet electromigration reliability requirements the UBM should contain a good reaction barrier layer. Cu is one of the common surface finishes. Cu is wet well by solder, but the interfacial reaction of Cu and Sn based Pb-free solder is aggressive. In addition Cu diffuses rapidly under an electric current. Cu rapidly converts into a Cu—Sn intermetallic composition (IMC) under thermal and EM stress, resulting in poor reliability.
Ni UBM is widely used to improve Cu UBM. Although Ni showed slower interfacial reaction than Cu, Ni UBM consumption is faster in Pb-free solder joint, especially when Ni UBM is used as a surface finish for both the chip and substrates (Cf. FIGS. 2, 3). Various Ni barrier layers over Cu UBM were extensively evaluated for this purpose (S. K. Kang, M. G. Cho, D-Y Shih, S. K. Seo, and H. M. Lee, “Controlling the Interfacial Reactions in Pb-free Interconnections by Adding Minor Alloying Elements to Sn-rich Solders,” Proc. 58th ECTC, Orlando, Fla., May, 2008, pp. 478-484, (2008)). Due to aggressive interfacial reactions of Sn-rich solders during multiple reflows and high current EM tests, however, most Ni barrier layers investigated do not provide adequate protection in terms of the interfacial reactions and EM resistance.